One of the most important considerations in the design of gas turbine engines is to ensure that various components of the engine are maintained at safe operating temperatures. This is particularly true for elements of the combustor and turbine, which are exposed to the highest operating temperatures in the engine.
In the turbine of gas turbine engines, for example, high thermal efficiency is dependent upon high turbine entry temperatures. These entry temperatures, in turn, are limited by the heat which the materials forming the turbine blades and nozzle guide vanes can safely withstand. In addition to improvements in the types of materials used to fabricate these components, continuous air cooling has been employed to permit the environmental operating temperature of the turbine to exceed the melting point of the materials forming the blade and nozzle guide vanes without affecting their integrity.
A number of techniques are used in an attempt to effectively and uniformly cool the components of the turbine, combustor and other portions of gas turbine engines. The turbine nozzle segments, for example, are conventionally cooled by a combination of air impingement, film, pin fins, convection/film holes and thermal barrier coatings. Each nozzle segment, which comprises inner and outer bands interconnected by fixed nozzle guide vanes, is subjected to a combination of such cooling methods to reduce both the internal and external temperature of the bands and nozzle guide vanes.
One problem area in the cooling of turbine nozzle segments, and other components of the gas turbine engine, is at the joint connections between abutting nozzle segments. In order to prevent thermal hoop stresses, the inner and outer bands supporting the nozzle guide vanes must be segmented, i.e., a number of turbine nozzle segments each having arcuate-shaped inner and outer bands extend circumferentially about the turbine case and abut one another at their side edges. Conventionally, a slot or pocket is formed in the abutting side edge of adjacent turbine nozzle segments and a sealing member extends between the slots of abutting segments to create a seal therebetween. It has been found that this sealing area between abutting segments is cooled less effectively than the remainder of the inner and outer bands of the nozzle segment, which creates an uneven heat distribution along the nozzle segments.
Attempts have been made to improve cooling of the joint connection or seal area between abutting turbine nozzle segments, but problems have been encountered with each design. One design depends on the conduction of heat from the seal area to areas of the inner and outer bands which are impinged with air. Film cooling, i.e., the passage of cooling air closely adjacent the surface of the inner and outer bands, has also been utilized to cool the seal area. Still other designs depend upon leakage of air past the seals to achieve the necessary cooling in the seal area. Conduction of heat to areas of the inner and outer bands which are impinged with cooling air, and film cooling of the seal area, have both proven ineffective to adequately cool the seal region. While the leakage of cooling air past the seals can be sufficient to provide the required cooling, such air leakage is unevenly distributed along the abutting side edges of the nozzle segments and the inner and outer bands thereof can become very hot at localized areas therealong, particularly where the seal is firmly seated and prevents the movement of cooling air therepast.
Another technique which has been suggested to cool the seal area between abutting nozzle segments includes the formation of convection holes between the seal region and the side of the inner and/or outer bands which are impinged by cooling air. Depending upon the temperature of the gases at which the gas turbine engine operates, a relatively large number of convection holes are required. The drilling of such a large number of holes is expensive, and location tolerances are difficult to hold. Additionally, large numbers of convection holes could weaken the part by producing localized stress concentrations thereat. Moreover, such convection holes can produce discontinuities in the thermal barrier coating applied to the hot or gas side of the inner and outer bands of the nozzle segments which reduces the effectiveness of the thermal barrier coating.